I have diverticulitis and am currently on a low residue diet. Is milk a low residue drink?

Yes. If you temporarily are on a low fiber diet, the easiest way to follow that diet is no fruits, no vegetables, no whole grains! meat, fish, chicken and dairy are fine. You may have white rice and white bread. Only exceptions to fruit/veggies is mashed potatoes and non chunky apple sauce. Not a healthy diet in the long run but necessary in the short(4-6weeks) to allow the colon to heal and recover.
Yes, but... Yes it is, but if you are lactose intolerant you should avoid milk. A low residue diet contains a low amount of fiber. Fiber causes gas production in the colon-- so does milk if you are lactose intolerant.